Currently known processes for sulfur dioxide from oxygen-containing gases typically employ caustic processes (e.g., Amerex, Belco, Croll-Reynolds, Envirochem, Turbosonic), or non-caustic processes (e.g., Cansolv, ClausMaster, CrystaTech). While the capital investment for a caustic process is generally low, operating and disposal costs may be prohibitively expensive. On the other hand, all or almost all of the currently known non-caustic processes are more capital intensive and are frequently not commercially proven.
For example, sulfur dioxide is removed in some of the known configurations using a caustic process in which gaseous sulfur compounds are converted into soluble sulfite/sulfate compounds. Typical examples of such configurations are shown in U.S. Pat. Nos. 3,719,742 to Terrana et al. and 3,790,660 to Earl et al. However, most of such configurations have a relatively high stripping steam requirement and are therefore economically less attractive. Other known caustic processes are described, for example, in U.S. Pat. No. 3,920,794 to La Mantia et al. Here, NaOH and Na2CO3 scrubbing solutions remove SO2 from gas streams. After the adsorption or scrubbing step, an oxidation step is performed to convert sulfites to sulfates by addition of catalytically effective metals (e.g., Fe, Cu, Co, Mn, and/or Ni). While such oxidation is relatively simple and effective, salts need to be added, and a secondary oxidation step may be required, especially where the level of sulfites in the scrubbing solution after adsorption of SO2 is relatively high.
To overcome at least some of the problems associated with caustic solutions, various alkanolamines (e.g., aqueous solutions of triethanolamine) can be used to absorb SO2 from a waste gas as described for example, in U.S. Pat. No. 3,904,735 to Atwood et al. However, several difficulties remain. Among other things, many alkanolamines have a relatively low selectivity towards SO2 and tend to absorb significant quantities of CO2. Still further, at least some of the alkanolamines exhibit relatively high evaporative losses and often promote oxidation of SO2 to SO3 where oxygen is present.
In still further known non-caustic processes, as described in U.S. Pat. No. 4,634,582 to Sliger et al., SO2 is removed from a waste gas stream by absorption in a buffered aqueous thiosulfate and polythionate solution, followed by regeneration of the enriched solution with hydrogen sulfide to form sulfur. Hydrogen sulfide recovered from the regeneration step is then introduced to the absorption step to reduce bisulfite concentration in the enriched solution. While such desulfurization is conceptually relatively simple, maintenance of the buffered solution often limits the capacity of such systems in at least some instances.
Alternatively, as described in our International patent application (published as WO 03/045544), sulfur dioxide-containing waste gas is introduced into a reducing gas generator that is operated using natural gas, air, and hydrogen to supply sufficient reducing gas to the effluent gas. Typical operation conditions are selected such that the oxygen is substantially completely removed from the waste gas, operation temperatures will generally be between about 1000° and 1500° F. The so formed hydrotreated feed gas comprises hydrogen sulfide, which is removed using a contactor. Such configurations advantageously improve sulfur removal under most conditions. However, high temperature operation and supplemental fuel gas are generally needed, which tends to increase cost and complexity of the operation.
Thus, although numerous configurations and methods are known in the art to reduce sulfur concentrations in oxygen-containing effluent streams, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and configuration to reduce the sulfur content in such streams.